When you're about to score but someone ruins it for you.
Can be ruined for one or both parties.
Mood killer.
Best described with an example.

Haley and her boyfriend Andrew were having phone sex when Andrew's brother James walked in.

Andrew: Then I'll take off your shirt and your bra and bite your neck and pull your hair
Haley: I'll take off your shirt and your pants and get on top of you and start biting your neck softly
Andrew: Then I'll move down to your stomach and take your panties off and I'll start to eat out your pussy

Haley: Next I'll take your boxers off and kiss your dick, I'll get on my knees and start sucking your cock
Andrew: Mmmmmm, Baby, what's next?
Haley: I think that's up to you;)
Andrew: I want you to suck me until I'm rock hard and about to explode. Then I'll get on top of you and pin you to the bed and start to put rock hard cock in your tight, wet....
James: Hey man, what's up?
Andrew: You fucking cock block!!!
James: Who are you talking to?
Andrew: Haley!!! We were having a good time!!
Haley: Tell you're brother he fucking ruined it!!!
James: YOU'RE WELCOME
Andrew: Get out James!!! Baby, I'm not horny anymore. I'm sorry my brother's a fucking cock block.
Haley: Ugh, neither am I. Tell him that I'm fucking pissed at him.